# std::signbit

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< cpp‎ | numeric‎ | math

Defined in header `<cmath>` |  |
| (1) |
bool signbit( float num ); bool signbit( double num ); bool signbit( long double num ); |  | (since C++11) (until C++23)
constexpr bool signbit( /*floating-point-type*/ num ); |  | (since C++23)
SIMD overload (since C++26) |  |
Defined in header `<simd>` |  |
template< /*math-floating-point*/ V > constexpr typename /*deduced-simd-t*/<V>::mask_type signbit ( const V& v_num ); | (S) | (since C++26)
Additional overloads |  |
Defined in header `<cmath>` |  |
template< class Integer > bool signbit( Integer num ); | (A) | (since C++11) (constexpr since C++23)

1) Determines if the given floating point number num is negative. The library provides overloads for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)

S) The SIMD overload performs an element-wise `std::signbit` on v_num. (See `math-floating-point` and `deduced-simd-t` for their definitions.) | (since C++26)

A) Additional overloads are provided for all integer types, which are treated as double.

### Parameters

num | - | floating-point or integer value
v_num | - | a data-parallel object of std::basic_simd specialization where its element type is a floating-point type

### Return value

1) true if num is negative, false otherwise.
S) A data-parallel mask object where the ith element equals true if v_num[i] is negative or false otherwise for all i in the range `[`​0​`,` v_num.size()`)`.

### Notes

This function detects the sign bit of zeroes, infinities, and NaNs. Along with std::copysign, `std::signbit` is one of the only two portable ways to examine the sign of a NaN.

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their argument num of integer type, std::signbit(num) has the same effect as std::signbit(static_cast<double>(num)).

### Example

Run this code

    #include <cmath>
    #include <iostream>

    int main()
    {
        std::cout << std::boolalpha
                  << "signbit(+0.0) = " << std::signbit(+0.0) << '\n'
                  << "signbit(-0.0) = " << std::signbit(-0.0) << '\n'
                  << "signbit(+nan) = " << std::signbit(+NAN) << '\n'
                  << "signbit(-nan) = " << std::signbit(-NAN) << '\n'
                  << "signbit(+inf) = " << std::signbit(+INFINITY) << '\n'
                  << "signbit(-inf) = " << std::signbit(-INFINITY) << '\n';
    }

Output:

    signbit(+0.0) = false
    signbit(-0.0) = true
    signbit(+nan) = false
    signbit(-nan) = true
    signbit(+inf) = false
    signbit(-inf) = true

### See also

abs(float)fabsfabsffabsl (C++11)(C++11) | absolute value of a floating point value (\(\small{|x|}\)|x|) (function)
copysigncopysignfcopysignl (C++11)(C++11)(C++11) | copies the sign of a floating point value (function)
C documentation for signbit

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